Formulation-dependent physicochemical properties and biological response of DOPC liposomes encapsulating Opuntia stricta extracts

The development of liposomes for the delivery of chemically labile natural extracts is often limited by extract instability and poorly controlled biological responses. In this study, zwitterionic 1,2-dioleoyl- sn -glycero-3-phosphocholine (DOPC) liposomes were engineered as platforms for the encapsulation of aqueous extracts from Opuntia stricta var. stricta (OpS) and var. dillenii (OpD), and the role of the formulation medium as a key design parameter was systematically investigated. Liposomes were prepared at pH 5.0 in water, phosphate solution, or acetate buffer and characterised in terms of size, surface charge, morphology, encapsulation efficiency, and loading capacity. While all formulations displayed nanometric dimensions, the ionic composition of the preparation medium markedly influenced liposome organisation and extract retention. Acetate-based formulations showed enhanced encapsulation efficiency (up to 61%) and loading capacity (up to 23%), together with improved size homogeneity compared to phosphate-based systems. Release studies under physiological conditions revealed a sustained and biphasic release profile for both extract varieties. The biological performance of liposomes was evaluated in vitro using human HaCaT keratinocytes and SH-SY5Y neuroblastoma cells. All formulations were cytocompatible towards HaCaT cells, whereas acetate-formulated OpS-Lip and OpD-Lip induced a moderate, formulation-dependent reduction in SH-SY5Y cell viability, highlighting the influence of physicochemical design parameters on cellular response. Overall, these results demonstrate that formulation conditions critically govern the colloidal and biological performance of DOPC-based liposomes, providing design guidelines for the development of lipid carriers for unstable plant-derived bioactives.